The spectroscopy technology has various advantages, such as environmental benefits, pollution-free, no damages to samples, fast detection speed, simultaneous quantitative analyses of multiple components, no need for any reagents or test paper, continuous and real-time monitoring, or the like. It is a real nondestructive detection technique. In the biomedical field, near-infrared spectroscopy has been employed to realize rapid and non-invasive detection of biochemical parameters such as degree of blood oxygen saturation. Near-infrared spectroscopy is considered as one of the most promising non-invasive techniques for some chemical components' detection in human bodies.
In component concentration detection by spectroscopy, substances to be detected are usually complex samples without being subjected to pre-processing such as refinement, or certain substances in a human body. The concentration of a component can be determined by establishing a prediction model using a multi-variable regression method when the component have obvious absorption characteristic on a given waveband. However, there may be many unpredictable interferential factors in the complex samples or bodies. For example, the absorption spectrum of some components in the human body may be seriously influenced by body temperature or mood and thus do not have obvious absorption characteristics on the given waveband. Thus, the concentration measurement and analysis have to be performed after the influence of such unpredictable background is removed. The spectral data from the sample with various backgrounds should be included in a training data set for establishing a robust mathematical model in the near-infrared spectroscopy.
In non-invasive detection of blood glucose concentration, for example, a signal produced by variations of the blood glucose concentration is very weak because the concentration in a body tissue is low and has small variations in a physiological range. On the other hand, extraction of the blood glucose signal is difficult due to light scattering, in addition to light absorption, when the light passes through the body tissue, because the body tissue has complex optical properties. Near-infrared light absorption also exists in water, fat, protein, or the like in the tissue, which may produce signals stronger than that produced by glucose concentration and its variations. Furthermore, a same chemical bond may have multiple absorption peaks from its fundamental frequency absorption or multiplication frequency absorptions distributing on different wavelengths in the given waveband region. And the different molecules in the sample or the body tissue also have various chemical bonds, which may make complicate overlap absorptions in the given near-infrared waveband. Moreover, concentration of substances in the body tissue may also be influenced directly or indirectly by factors such as metabolism, physiological period, and mood fluctuation of a living body, environment, or the like, which makes it difficult to extract the blood glucose concentration information from the recorded optical signals. It is difficult to monitor variations of these factors in real time. Therefore, a reference-based measurement may be employed to solve the above-described problem, because it could be considered as an effective approach to remove some background variations when deducting a background or reference signal from the original data. For example, relative measurement has been applied to realize the blood oxygen saturation measurement for clinical use by referencing to a signal recording at a baseline time.
In ex-vivo experiments, dual optical paths are usually applied to remove common interferences caused by drift of instruments or the like, when setting an analogue for the detection object on the reference optical path. A differential operation is performed by deducting a spectrum from the analogue sample, which has optical properties close to that of the skin or object to be detected, as a background spectrum or a reference spectrum. Experimental results show that such a method can remove influence of the common variation in the measurements effectively.
However, in some applications, such as in the measurement of blood glucose concentration of the human body, it is difficult to find an analogue having optical characteristics close to the human skin tissue and meanwhile containing the skin's variation information during the measurement. Also, in clinical practice of near-infrared non-invasive blood glucose detection, it is difficult to find a body region or skin region with a constant glucose concentration or without any glucose inside as a possible reference position, because glucose exists throughout the whole body and the concentration thereof varies constantly. Thus, the spectral processing method for reducing the background reference applicable in the ex-vivo experiment cannot be used in in-vivo detection directly. Even though a standard reflection plate or a phantom having a reflectivity, which has an order close to that of a diffuse reflectivity of human skin, is introduced into the measurement system as the background reference to remove a part of influence of hardware system reference signals, it is still difficult to obtain the variation signal because light propagation on the reflection plate or the phantom is different from that on the human skin and factors such as metabolism, physiology period, and mood fluctuation of the human body, environment influence or the like cannot be reflected in the spectrum of the standard reflection plate or phantom. This appears to be a great challenge to seek an appropriate reference for performing the reference measurement of blood glucose using the near-infrared spectroscopy.
Thus, in detection of concentration of a particular component in the body tissue using the near-infrared spectroscopy, the measurement may be limited to the approaches for extracting the useful signals, which are disturbed by the varying body background, like the physiological fluctuation of the human body.
Kexin X U, et al., have invented a concentration measurement method using a floating reference position (see, CN patent application published as CN1699973A). A reference which can be used as “background” is found in information contained in the measured spectrum per se, as shown in FIG. 1. Variations of the concentration of the substance to be detected, such as a particular component in the human body, e.g., glucose, may cause variations of optical characteristics, such as absorption coefficient, scattering coefficient, or the like, of the substance to be detected. In a certain radial position rk from a light source, diffuse light energy is substantially equally varied by absorption and scattering of a tissue and thus remains substantially constant in spite of concentration variation of the glucose. The position rk is referred to as the floating reference position. The light intensity measured at this position can reflect influence of almost all interference factors other than the glucose concentration variation in the detection process. Thus, the spectrum at this position can be used as the “background” and be subtracted from the spectrum at other positions to extract their glucose specific information. This performing the reference measurement for in-vivo detection appears to be similar to that in the ex-vivo experiments. Existence of the floating reference position has been verified in Monte Carlo simulation and ex-vivo experiments.
However, the reference used as the “background”, or the floating reference position, may be different for different subjects to be detected, different skin regions of a subject to be detected, and different light wavelengths, because the light energy variation caused by the blood glucose concentration variation is associated with the optical characteristics of the media, i.e. the body issue.
In the Monte Carlo simulation, palm skin is used as a tissue object for measuring the human body blood glucose concentration. A three-layer skin model including epidermis, dermis, and subcutis is applied in the simulation. A number of incident photons set for the Monte Carlo simulation program is 109. Optical parameters of the three layers given by Maruo, et al. (see, Maruo K., Tsurugi M., Chin J., et al., Noninvasive blood glucose assay using a newly developed near-infrared system, IEEE Journal of Selected Topics in Quantum Electronics, 2003. 9(2): p. 322-330; Maruo K., Oota T., Tsurugi M., et al., New methodology to obtain a calibration model for noninvasive near-infrared blood glucose monitoring, Applied Spectroscopy, 2006. 60(4): p. 441-449) are based on. The optical parameters of the dermis of the skin model vary while the optical parameters of the other layers remain constant when the glucose concentration increases from 0 to 500, 1000, and 1500 mg/dL, respectively. A typical diffuse spectrum distributed across a radial distance of a detector from a light source can be obtained in a wavelength range of 1200-1700 nm when the epidermis, the dermis, and the subcutis of the three-layer skin model is set to 0.5 mm, 3.5 mm, and infinity, as shown in FIG. 2. The distribution is close to exponential distribution, in which diffuse reflection light intensity reduces quickly as the radial distance increases. The diffuse reflection light intensity beyond the radial distance of 2.0 mm is relatively weak, which decreases from 10−1 to 10−8.
Radial distributions obtained by subtracting the diffuse reflection light intensity at 0 mg/dL from the diffuse reflection light intensities at different glucose concentrations are shown in FIGS. 3 (a1))-(a3), which correspond to wavelengths of 1200 nm, 1300 nm, 1400 nm respectively, and a float reference positions' distribution on 1200-1400 nm shown in FIG. 3(a4) thereof. It is clear from the figures that there is a radial position where the diffuse reflection light intensity is insensitive to the glucose concentration variation at respective wavelengths for the given tissue media. Obviously, this position is the floating reference position, and it is dependent on wavelength. The floating reference position changes slightly in a wavelength range of 1200-1300 nm, especially for the shorter wavelength band of 1200-1260 nm. The reference position changes obviously in a wavelength range of 1300-1400 nm. The reference position appears to be closer to the light source as the wavelength becomes longer. The floating reference position does not exist for wavelengths greater than 1400 nm for this media case.
Even at one same wavelength, there may be differences in tissue components for different measurement subjects or media. Also, the tissue components in one same measurement media may change with time. That is, the optical characteristics of a tissue may change. FIG. 3(b) shows how the floating reference position changes at 1300 nm for the three-layer tissue while the absorption coefficient μa and the scattering coefficient μs change at a gradient of ±20%. The initial optical parameters including μa and μs were applied when the initial body blood glucose concentration was set to 100 mg/dL in the media. As shown in the figure, scattering coefficient varying, which may be induced by different subjects or the different skin tissue regions of a same subject or the same tissue but measured at different conditions etc., could affect the floating reference position obviously.
As a result, the floating reference position may vary at a given wavelength when the tissue layers' thicknesses are varying. FIG. 3(c) shows how the floating reference position changes in the three-layer skin model at the wavelength of 1300 nm while the thickness of palm epidermis changes in a range of 0.1-1.0 mm and the thickness of the dermis changes in a range of 2.0-4.0 mm. The initial body blood glucose concentration was also set to 100 mg/dL in the simulated media. The floating reference position moves away from the light source as the epidermis thickness increases because the epidermis thickness has a great influence on the diffuse reflection light intensity.
Thus, it is impossible to cover different measurement subjects, a same measurement subject in different states, or a plurality of wavelengths, or even measurement errors may be caused, if a constant radial position is applied as a reference position for the measurement. Therefore, it is desired to develop a more general and robust measurement method suitable for different measurement subjects and wavelengths.